The present invention relates generally to a line-narrowed gas laser system, and more particularly to a gas laser system such as an F2 laser system in which laser beams are narrowed to a spectral linewidth of 0.2 pm or lower and a spectral purity of 0.5 pm or lower while ASE (amplified spontaneous emission) is substantially cut off from the laser beams.
The present invention is also concerned with a gas laser system using laser gases inclusive of F2, for instance, a KrF or ArF laser system, in which the ASE is cut off from laser beams, thereby making spectral linewidths and spectral purities by far narrower.
As semiconductor integrated circuits become increasingly fine with increasing packing densities, aligners used for their fabrication are now required to have stronger resolving power than ever before. It is in turn required to make shorter the wavelengths of exposure light emitted from exposure light sources. To this end, semiconductor aligner light sources are changed from conventional mercury lamps over to 248-nm wavelength KrF lasers, and 193-nm wavelength ArF lasers are being used as light sources for much shorter wavelengths.
For photolithographic technologies to achieve on semiconductors a semiconductor integrated circuit having a linewidth of 70 nm or less, there are demanded exposure light sources of wavelengths of 160 nm or less. F2 (molecular fluorine) laser systems that give out ultraviolet rays of wavelengths of around 157 nm are now thought of as a promising candidate for those light sources, and the tempo of research and development aimed at mounting those light sources on aligners is quickened. Thus, the present invention is directed to performance improvements in KrF and ArF laser systems and performance improvements in F2 laser systems with a view to mounting them on semiconductor aligners.
The optical technology is roughly broken down into the following two types that use:
1) a dioptric system, and
2) a catadioptric system.
A typical dioptric system is a projection optical system commonly used with prior art aligners. One grave problem with photolithography is how to correct chromatic aberrations in an optical system. With the dioptric system, correction of chromatic aberrations has been achieved by combinations of lenses or other optical elements having different kinds of refractive indices. By virtue of some limitations on the types of available optical materials transparent to a wavelength range in the neighborhood of 157 nm, however, there is now no option but to use CaF2 (fluorite).
The use of the catadioptric system for photo-lithography ensures that chromatic aberrations are reduced by using a reflecting optical element having no chromatic dispersion in combination with a refracting optical element. For this reason, an aligner using such a catadioptric system holds great promising in a wavelength range of the order of current 157 nm. However, the catadioptric systems are not so often used as conventional dioptric systems by virtue of difficulties with the optical axis alignment of aligners.
There is one promising means for making the dioptric system commonly used in the prior art compatible with the wavelength range of around 157 nm. That is to use as an aligner light source an F2 laser system designed to give out line-narrowed laser beams.
Although depending on running conditions such as the total pressure of a discharge gas or the like, the spectral full width at half maximum (FWHM) of an F2 laser beam is of the order of 1.5 to 1.2 pm when its bandwidth is not narrowed (in free running operation modes). For the dioptric system, this spectral full width at half maximum must be narrowed down to 0.2 pm or less. For KrF and ArF laser systems, too, the bandwidth of laser beams must be narrowed because their full width at half maximum (FWHM) on free running operation is as wide as several hundred nm. The present invention is directed to the line-narrowing technique.
One exemplary construction of a line-narrowed F2 laser system using one or more expanding prisms and a diffraction grating is shown in FIG. 1. It is noted that a line-narrowed KrF or ArF laser system, too, has a similar construction.
A laser chamber 1 is filled therein with an F2 laser-inducing medium gas (hereinafter called the laser gas). As high-voltage pulses are applied from a high-voltage pulse generator 3 to a pair of electrodes 2 provided in the laser chamber 1 and located oppositely at a given spacing, discharge is generated between the electrodes 2 so that the laser gas is excited at a discharge portion. The excited laser gas gives out seed light turning to a laser beam. Within the laser chamber 1 there are further provided a fan 4 and a radiator (although not shown). The laser gas is circulated within the laser chamber by the fan 4, and the laser gas heated by discharge to a high temperature is cooled down by way of heat exchange with the radiator. As shown in FIG. 1, the laser chamber 1 includes windows 5 in which window members, each in a -shaped form, are mounted at a Brewster's angle or a parallel Brewster's angle. For the electrodes 2, an anode electrode and a cathode are located at a given spacing in the vertical direction to the paper.
A laser resonator is built up of a diffraction grating 8 mounted on a line-narrowing module 6 described layer and an output mirror 9.
The aforesaid seed light that turns to a laser beam makes round trips between the line-narrowing module 6 including a diffraction grating 8 and a magnifying prism 7 and the output mirror 9 by way of the discharge portion, and is extracted as the laser beam from the output mirror 9.
A part of the laser beam emerging out of the output mirror 9 is introduced through a beam splitter 10 into a wavelength monitor 11, where the output, center wavelength, etc. are measured.
Line-narrowing occurs through the optical line-narrowing module 6 having a spectral function, which is located within the laser resonator. For instance, the line-narrowing module 6 is made up of a casing and a diffracting grating 8 and an expanding prism 7 located in the casing, and spectral line-narrowing is achievable through wavelength selection by the diffraction grating 8.
It is possible to vary the center wavelength of oscillation by rotation of either one of the diffraction grating 8 and the expanding prism 7.
It is noted that when a highly reflective mirror is located at any position between the laser chamber 1 and the diffraction grating 8, it is also possible to rotate the highly reflective mirror to vary the angle of incidence of light on the diffraction grating 8, thereby varying the center wavelength of oscillation.
Wavelength control is implemented by rotation of any one of the diffraction grating 8 and the expanding prism 7 in the line-narrowing module 6 or the highly reflective mirror located at any position between the laser chamber 1 and the diffraction grating 8 (although not shown) in response to center wavelength signals from the wavelength monitor 11.
Even when such an optical line-narrowing module having a spectral function in the laser resonator is used as the line-narrowing means for a line-narrowed F2 laser system, it is still difficult to narrow its spectral linewidth (FWHM) to 0.2 pm or less demanded for the dioptric system of an aligner.
Here let Δλ be a spectral linewidth, W be the width of a light beam incident on the diffracting grating and θ be the blaze angle (=Littrow angle) of the diffraction grating. Then, the relations areΔλ∝ cos θ/WThat is, the larger the blaze angle θ of the diffraction angle and the wider the width of the light beam incident on the diffraction angle, the narrower the spectral linewidth Δλ becomes.
To increase the light beam width W, it is necessary to increase the expanding factor of the expanding prism or the number of expanding prisms and widen the width of the diffraction grating. When it comes to an aligner light source, however, there are some limitations on system size at a worksite and, hence, some limitations on the size of a line-narrowing module used thereat. For the reason that the light transmittance of an expanding prism is not 100% with respect to 157-nm wavelength light, the more the expanding prisms, the lower the oscillation efficiency becomes. Thus, there are limits to increasing the number of expanding prisms and the width of the diffraction grating.
There are also limits ascribable to optical part fabrication techniques. For instance, the blaze angle cannot possibly be larger than a predetermined value because of limits to diffraction grating fabrication.
Under such situations, optical line-narrowing has some limits.
Proc. SPIE Vol. 3679, (1999) 1030–1037 shows that as the laser pulse width increases, the spectral linewidth of laser light becomes narrow. This has actually been demonstrated through experimentation by the inventors.
In other words, to achieve further line-narrowing beyond the aforesaid limits to optical line-narrowing, it is required to stretch the pulse of a laser beam (pulse stretching).
Even with pulse stretching, however, it is still difficult to narrow the spectral linewidth (FWHM) to 0.2 pm or less.
This is for the following reasons. Like excimer lasers (KrF, ArF, XeCl, etc.), molecular fluorine F2 lasers have high gains. When it comes to a laser system, high gain means that light emerging from the output mirror contains much light without resonated in a line-narrowing module (amplified spontaneous emission (ASE) or, in another parlance, parasitic oscillation light). The ASE is light that is emitted from the output mirror with no round trip in the laser resonator, and hardly subjected to line-narrowing probably because it has not passed, or passed only once, through the line-narrowing module. A laser beam given out of a conventional line-narrowed F2 laser system contains the ASE component that makes it difficult to narrow the spectral linewidth (FWHM) of the laser beam to 0.2 pm or lower. Why the ASE occurs is now explained in detail.
FIG. 2 is illustrative of the progressions over time of the waveform of sidelight upon laser oscillation (hereinafter called simply the sidelight), the waveform of laser pulse and spectral linewidth oscillated from a prior art F2 laser system in which the spectral linewidth (FWHM) cannot be narrowed to 0.2 pm or less. It is here noted that these waveforms were obtained with a laser resonator formed of a diffraction grating and an output mirror and having a length of 1,500 mm.
Here the light generated by a laser gas excited by discharge occurring between a pair of electrodes is called the “sidelight”. The sidelight is observed from a position that is not located on the laser resonator (for instance, the electrode side position in a substantially vertical direction to the longitudinal direction of the electrodes).
The waveform of the sidelight is indicative of a gain distribution over time of the laser beam. In other words, the sidelight is indicative of a gain distribution upon laser oscillation.
A laser pulse rises sharply beyond a threshold value upon the sidelight reaching a peak. In other words, main laser oscillation (not ASE oscillation) rises sharply from the starting point defined by the first peak of the sidelight.
In the laser pulse waveform, one peak is observed at a position after 20 ns from the discharge excitation start (0 ns). A spectral linewidth of this peak position at a time A was much the same as that on free running operation. Regarding to FIG. 2, it is noted that the ordinate as spectral linewidth is not linear, and a spectral linewidth at the time A is actually considerably large, although it looks as an about 0.4 pm spectral linewidth.
As already described, an F2 laser system has high gain as in the case with excimer lasers (e.g., KrF, ArF, XeCl lasers). In a laser system having high gain, as gain rises and goes beyond a given value (that is, a given time goes by after the rise of gain), oscillation (ASE) occurs by light that makes one single pass through the resonator without making round trips therein.
A peak at the time A of the laser pulse waveform shown in FIG. 2 was also observed in a misalignment state where the optical axis of the laser resonator was displaced. Accordingly, the light for the first peak portion of the laser pulse waveform is thought of as the ASE. As already described, the ASE is light that is emitted out of the output mirror without making round trips in the laser resonator, and is hardly subjected to line-narrowing probably because it has not passed, or passed only once, through the line-narrowing module.
The light that is not extracted as the ASE makes round trips in the laser resonator and subjected to line-narrowing, leaving the laser system as a laser beam.
As shown schematically in FIG. 3(a), the spectral linewidth of one laser pulse is the integral over time of each spectral linewidth at each point of time in the laser pulse, and so when the ASE is given out in an early state of the laser pulse, the result is that it is difficult to narrow the spectral linewidth to 0.2 pm or lower. That is, as shown schematically in FIG. 3(b), the ASE having a spectral linewidth of 0.6 pm or greater is superposed on the spectral properties of the laser pulse. This in turn causes the overall integration spectrum of the laser pulse to have a spectral linewidth of greater than 0.2 pm even though the spectral linewidth of main laser oscillation is somehow not greater than 0.2 pm.
With the laser pulse waveform containing the ASE, on the other hand, it is difficult to satisfy the specifications of an aligner light source in terms of spectral purity.
The “spectral purity” used herein is understood to refer to an index to the degree of concentration of spectral energy, indicating a linewidth including a “certain area ratio” of a spectral waveform. For instance, a commonly used “95% purity” refers to a linewidth that accounts for 95% of the entire area of that spectral waveform, as measured from its center side. The spectral purity usually needed for a dioptric type of photo-lithographic light source is 0.5 pm.
As already described, the ASE is light that is hardly subjected to line-narrowing. Even though the pulse of a laser beam is somehow stretched, the hem form of the spectral integration waveform shown in FIG. 3 remains invariable as long as the ASE component exists. It is thus impossible to satisfy the specifications regarding the spectral purity although depending on what is demanded for an aligner. While the values of the spectral linewidth and spectral purity demanded vary with wavelenth, it is understood that such problems as mentioned above arise with KrF and ArF laser systems.